On Jan. 25, 1998, scientists monitoring undersea volcanoes were alerted
to a sudden swarm of earthquakes, which occurred as magma began moving
toward the surface and into the south rift zone on the Pacific Ocean floor.
The lava flow then inflated and drained out over the next two hours. In
the following days, the entire volcano summit gradually subsided as magma
oozed out of the summit reservoir and spread over the seafloor. This eruption
of the undersea Axial Volcano on the Juan de Fuca mid-ocean ridge off the
coast of Washington state spewed lava that engulfed a seafloor instrument
placed on the volcano to measure pressure and vertical deformation of the
seafloor.

The Volcanic System Monitor (VSM), also known as a rumbleometer, was
resting on the caldera’s south rift zone. It survived the eruption and
is here to tell the tale, for the first time providing a glimpse into the
heart of one of Earth’s most powerful and least understood phenomena.

“The instrument was simply in the right place at the right time, with
the right sensors, and happened to have the right physical design to survive
the eruption,” write Christopher Fox, William Chadwick and Robert Embley,
all with NOAA’s Pacific Marine Environmental Laboratory (PMEL) Vents Program,
in the Aug. 16 Nature. PMEL scientists rescued the lost rumbleometer 18
months after the volcano erupted. They found it in three feet of solidified
lava on the ocean floor, at a depth of 1,525 meters.

As is common during basaltic volcano eruptions, the pattern observed
by the VSM was a brief burst at a high effusion rate, followed by a longer
period of waning rates, and then subsidence during lava withdrawal. Even
though the entire inflation and drainout cycle lasted only two hours, the
basaltic lava spread over the ocean floor to create a lava plain more than
350 meters wide.

When the frigid ocean water met the thinly spread sheet of lava, a solid
upper crust formed, serendipitously entombing the rumbleometer in solid
crust while the molten interior continued flowing. The temperature near
the rumbleometer peaked at 9.5 degrees Celsius, only 7.5 degrees Celsius
above the normal water temperature of 2 degrees Celsius, even though the
temperature of the erupting lava was 1200 degrees Celsius.

“Our principal focus is to see how undersea eruptions affect the
ocean environment,” Fox says. “We are also interested in the basic research
questions of how deep-sea volcanoes work, their time scales, dynamics and
so on.”

Before the rumbleometer’s fortuitous entrapment, scientists had only
been able to survey and observe undersea eruptions after they happened.
The VSM recorded the duration, character and effusion rate of this eruption
while it happened — and made the first in situ ground deformation measurements
of a submarine eruption.

The data also helped researchers begin to unravel a few deep-sea mysteries.
They can now calculate the amount of magma withdrawn during the eruption
and the depth of the magma reservoir beneath the caldera. And the pressure
measurements, which document the change in the depth of the instrument
during the eruption, provide, for the first time, the means to directly
estimate the effusion rate for a submarine eruption.

Submarine volcanoes are the primary sites for the exchange of heat and
chemicals from Earth’s interior to its surface. Scientists have just begun
to understand undersea volcanoes, as they have only been able to detect
submarine eruptions for the last decade. Their primary detection tool has
been SOSUS, a classified U.S. Navy Sound Surveillance System, currently
“the only means we have to monitor volcanic activity in the deep sea in
real time,” Fox says.

“Our primary goal now is to directly observe a deep-sea eruption in
progress,” Fox says. “This will require pre-positioning instruments on
Axial Volcano that can be commanded from shore to investigate active eruptions.

“The ocean is an extremely active part of the earth that still harbors
many unknowns. We have a lot of new technology to work with in the
deep sea and are attempting to unravel some of these mysteries.”